select @@global.datetime_format;
@@global.datetime_format
%Y-%m-%d %H:%i:%s
select @@session.datetime_format;
ERROR HY000: Variable 'datetime_format' is a GLOBAL variable
show global variables like 'datetime_format';
Variable_name	Value
datetime_format	%Y-%m-%d %H:%i:%s
show session variables like 'datetime_format';
Variable_name	Value
datetime_format	%Y-%m-%d %H:%i:%s
select * from information_schema.global_variables where variable_name='datetime_format';
VARIABLE_NAME	VARIABLE_VALUE
DATETIME_FORMAT	%Y-%m-%d %H:%i:%s
select * from information_schema.session_variables where variable_name='datetime_format';
VARIABLE_NAME	VARIABLE_VALUE
DATETIME_FORMAT	%Y-%m-%d %H:%i:%s
set global datetime_format="foo";
ERROR HY000: Variable 'datetime_format' is a read only variable
set session datetime_format="foo";
ERROR HY000: Variable 'datetime_format' is a read only variable